This invention relates to high solids curable polymer resin coating compositions containing amino cross-linking agents and particularly to such compositions containing alkoxymethyl melamine cross-linking agents.
High solids polymer resin coating compositions cross-linked with etherified amino resins are finding increasing use for general industrial and appliance applications and in automotive top finishes. Such coating compositions when compared to lower solids coatings compositions often reduce solvent emission and are more acceptable from an environmental standpoint. In industrial applications when coatings are cured by baking, monomeric methoxymethyl aminoplast resins have achieved widespread acceptance as cross-linking agents. While it has been found that the higher cross-link density achieved more quickly with the chemically efficient methoxymethyl aminoplast resin results in highly resistant films and finishes, it has also been observed that recoating of a baked coating with the same coating system is very difficult. Without any sanding, etching, or other treatment of the already coated part no effective intercoat adhesion is achieved. In many coating operations, recoating is necessary for various reasons. For example, coated parts may be damaged during assembly or multiple coatings as in the case of two-tone colored automobiles may be desired. In multiple coating operations, the part is first coated with one color which is subsequently baked. The area not to be painted with a second color is masked and then the second color is applied on the remaining uncovered area of the part and cured. Although some high solids coating compositions cross-linked with methoxymethyl aminoplast can function satisfactorily in such an operation under controlled laboratory bake conditions, in actual operation they fail because of decreased control of the curing cycle. It has been found that baking of the first coat at higher temperature for longer time periods during production than should normally occur reduces the intercoat adhesion of the second coat with more pronounced losses occurring at low temperature re-bake conditions. It has been shown that intercoat adhesion improves with the use of mixed ether melamine formaldehyde cross-linking agents. For example, mixed methylated n-butylated melamine formaldehyde resin has been shown to provide improved intercoat adhesion. In addition, U.S. Pat. No. 4,374,164 shows that a high solid composition containing a mixed methylated isobutylated melamine resin as cross-linking agent gives improved inter coat adhesion.
However, the mixed methylated isobutylated cross-linker requires about 2 to 3 isobutyls per melamine to effectively improve the intercoat adhesion of the coating system. The increased isobutyl content and the decreased methyl content per melamine nucleus cause the coating system to cure more slowly. The slower cure rate is indicated by a lower cross-link density and poorer weatherability as indicated by poorer UV stability and chemical resistance.
It has now been found that further improvement in intercoat adhesion can be achieved at low temperature re-bake conditions with a high solids composition using mixed alkyl ether aminoplasts containing C.sub.8 to C.sub.12 alkyl ether groups. Furthermore, a higher cross-link density can be achieved, since fewer C.sub.8 to C.sub.12 alkyl ether groups per aminoplast are required than isobutoxy to provide improved intercoat adhesion. The compositions comprise a curable polymer resin coating composition comprising
a. a hydroxy-containing resin and
b. A mixed ether aminoplast of degree of polymerization in the range of about 1 to about 3, comprising nuclei selected from the group consisting of melamine, acetoguanamine, adipoguanamine and benzoguanamine and attached thereto alkyloxymethyl groups in the range of about 2n-2 to about 2n per nucleus where n is the number of amino groups per nucleus, the alkoxymethyl groups being selected from C.sub.8 to C.sub.12 alkyloxymethyl and methoxymethyl groups in a ratio within the range of about 0.2 to about 0.5 C.sub.8 to C.sub.12 alkoxymethyl group per methoxymethyl group.